The invention relates to fused silica. More particularly, the invention relates to fused silica comprising hydrogen. Even more particularly, the invention relates to methods of loading fused silica with hydrogen.
Fused silica articles that will be exposed to 193 nm excimer laser radiation, such as lenses and other optical components, are usually loaded prior to exposure with hydrogen (H2) to mitigate the damage to the glass that is caused by the laser light. Such hydrogen loading typically involves establishing at least a minimum hydrogen concentration everywhere in the glass, while simultaneously keeping the average hydrogen concentration below a predetermined limit.
Hydrogen loading is accomplished by diffusing hydrogen into the glass in a furnace maintained at a temperature of less than about 500° C. The hydrogen concentration at or near the glass surface is determined by the combination of temperature and hydrogen partial pressure. Hydrogen concentration within the bulk of the fused silica is determined by diffusion rates and loading time. Since diffusion is slow, the time required to transport enough hydrogen through the glass to satisfy the minimum loading concentration everywhere in the fused silica may range from days to several months.
One method of loading fused silica with hydrogen combines temperature and pressure to set the H2 concentration at the surface equal to the average loading limit. These single pressure loading conditions need to be maintained long enough for sufficient hydrogen to diffuse into the center of the part to exceed the loading minimum. Because the hydrogen level anywhere other than at the glass surface would be less than the surface concentration, glass loaded this way automatically remains below the average loading limit. However, the single pressure loading process can be an extremely lengthy because the hydrogen concentration at the glass surface is never that high, and the rate of diffusion into the glass is therefore relatively slow.
In an attempt to decrease the time needed to load fused silica with hydrogen, a two-step loading cycle is currently used. By using an elevated hydrogen partial pressure during the first step, more hydrogen initially enters the fused silica part than in the single pressure loading process. The higher initial amount of hydrogen reduces the loading time by allowing more hydrogen to diffuse to the center of the part at an early stage of loading, thus reducing the overall loading time. However, because much more hydrogen initially enters the part, the average concentration limit may be exceeded well before loading the entire part to the minimum concentration. The hydrogen pressure is therefore dropped during the second step to decrease the average concentration by removing some of the excess hydrogen near the surface, even as the H2 concentration at the center of the part is still increasing. Compared to single pressure loading, such a two-step cycle is capable of reducing the loading time by more than 50%. Nonetheless, the time needed to load fused silica parts with a sufficient amount of hydrogen using the two-step process remains long.
In a production environment, it is often desirable to simultaneously load multiple fused silica parts having different thicknesses together in the same furnace. The two-step loading process also limits the range of thicknesses of fused silica parts that may be loaded together in the same furnace, as the time scale needed for the loading with hydrogen and subsequent unloading of the fused silica part depends on the thickness of the part.